html{
/* 默认像素为16px */
    font-size: 10px;
    font-size: 62.5%;
}
/* em是相对父容器字体大小 */
.ems li{
    font-size: 1.5em;
}
/* rem是相对根元素（html）字体大小*/
.rems li{
    font-size: 1.5rem;
}
/* 颜色值 */
.color_demo{
    /* color: red; */
    /* color: #ff0000; */
    /* color: rgb(255, 0, 0); */
    /* color: hsl(0, 100%, 50%); */
    color: rgba(255, 0, 0, 0.5);
    /* color: hsla(0, 100%, 50%,0.5); */}
    
.box{
    width: 300px;
    height: 300px;
    margin: 20px auto;
    border: 1px greenyellow;
    border-radius: .5em;
}

.bg-image{
    background-image: url(1.jpg);
    width: 1000px;
    height: 1500px;
}
.bg-gradient{
    background-image: linear-gradient(45deg,black,white );
}